To protect the circuit components of electronic equipment from potentially damaging electromagnetic radiation, such as an externally-sourced electromagnetic pulse (EMP) or other interference signals such as radar, broadcast radio and TV, Cellular Phone, etc., it is customary practice to house the equipment within some form of shielded structure such as a cabinet, or enclosure, for example. An adjunct to this shielding structure is the need to verify its shielding effectiveness, once the equipment has been deployed at a host facility. Up to the present, it has been conventional practice to conduct only ‘acceptance’ testing of the shielding for densely populated enclosures within a laboratory environment at the factory, and then assume that once it has passed the acceptance test, the shielding structure's effectiveness will be sustained in the equipment's deployed environment.
However, there is a government agency ‘verification’ requirement (MIL-STD-188-125) that mandates the ability to test the shielding effectiveness of the protective structure subsequent to deployment of the equipment at a host facility, and such testing can be difficult or impossible due to the lack of room inside a densely populated shielded structure. This strict verification requirement creates a two-fold problem that is typically encountered when attempting to conduct on-site testing of the electromagnetic radiation shielding-effectiveness of the protective enclosure.
Firstly, there is usually very little, if any, room inside the equipment cabinet to install testing hardware and its associated antenna, particularly once the cabinet has been integrated with other units at a host site, such as a commercial communication facility. Secondly, it is necessary that signals emitted by the testing apparatus not interfere with the operation of other electronic circuitry that may be located within the same environment as the electronic circuitry under test. Indeed, commercial telecommunication providers customarily refuse to allow the use of RF radiating test equipment in their facilities for fear that the testing might interrupt service.
A low profile, near field, radiation efficient decade bandwidth antenna is needed for implementing Electromagnetic Protection Test and Surveillance System (EPTSS) technology, such as for use with the system disclosed in U.S. Pat. No. 6,987,392 to Harris Corporation of Melbourne, Fla. EPTSS may require efficient RF radiation in close proximity to conductive surfaces and equipment inside relatively small shielded equipment enclosures. There is currently no commercially available antenna technology to meet all EPTSS requirements. There are presently no decade-bandwidth small antennas that radiate efficiently in close proximity to conductive surfaces.
Log Periodic antennas (LPA) have been used inside large shielded enclosures for shielding effectiveness tests, however their form factor is incompatible with small enclosures. Log periodic antennas operate over a broad frequency range. Generally log periodic antennas have a plurality of dipole elements in a planar spaced array. The length of the elements and the spacing between the elements are selected in accordance with a mathematical formula, with the shortest elements being near the top of the antenna. Feed conductors generally connect at the tip of the antenna. Electrical connections from feed conductors to opposed elements are alternated to provide a 180 degree phase shift between successive elements.
U.S. Pat. No. 5,093,670 to Braathen discloses a log periodic antenna formed by printed circuit board manufacturing methods onto an insulative substrate. The dipole elements and one feed conductor are formed on one side of the substrate and a second feed conductor is formed on the opposite side of the substrate. Vias through the substrate connect the second feed conductor to alternating opposed dipole elements.
U.S. Pat. No. 5,917,455 to Huynh et al. discloses an array of log periodic antennas mounted on a backplane. Each antenna includes two flat dipole strips of conductive material with bases of the dipole strips mounted to the backplane in a spaced configuration. Each antenna is fed by a coaxial feed line with the center conductor being connected to one dipole strip and the jacket conductor being connected to the other dipole strip.
Classic spiral antenna configurations may have a good form factor. For example, U.S. Pat. No. 4,309,706 to Mosko entitled “Wideband Direction-Finding System”, U.S. Pat. No. 4,525,720 to Corzine et al. and entitled “Integrated Spiral Antenna and Printed Circuit Balun”, U.S. Pat. No. 5,990,849 to Salvail et al. and entitled “Compact Spiral Antenna” and U.S. Pat. No. 6,067,058 to Volman entitled “End-Fed Spiral Antenna, and Arrays Thereof” disclose various spiral antennas, however they are physically too large for use with EPTSS, and are not optimal for near field applications.
Additional spiral antennas are also shown in U.S. Pat. No. 6,191,756 to Newham, U.S. Pat. No. 6,266,027 to Neel, U.S. Pat. No. 6,407,721 to Mehen et al. and U.S. Pat. No. 6,750,828 to Wixforth et al. These antennas may not meet all EPTSS requirements, and there are presently no decade-bandwidth small antennas that radiate efficiently in close proximity to conductive surfaces, such as for use with EPTSS.